Preheat your oven to 375°F (190°C) so it's ready for your delicious bake.

Melt butter in a skillet over medium heat, then add sliced sweet onions. Sauté until golden brown, about 15-20 minutes.

Thinly slice your potatoes, aiming for uniform thickness of about 1/8 inch.

In a bowl, combine heavy cream with salt, pepper, and fresh thyme. Fold in the caramelized onions.

In a greased baking dish, layer half of the sliced potatoes followed by half of the onion-cream mixture. Repeat the layers, then top with Gruyère cheese.

Cover with aluminum foil and bake for 45 minutes. Then, remove the foil and bake for an additional 15-20 minutes until the top is golden brown.

Let your bake rest for about 10 minutes after removing from the oven.